Mailpit has an incomplete fix for GHSA-6jxm: HTML check still permits SSRF to private/loopback/IMDS via missing IP-filter dialer

Summary The fix for GHSA-6jxm-fv7w-rw5j CVE-2026-23845, "Server-Side Request Forgery SSRF via HTML Check API", shipped in mailpit v1.28.3, hardened internal/htmlcheck/css.go::downloadCSSToBytes with a 5MB size cap, a text/css content-type check, login-info stripping in isValidURL, and an opt-in...